1. Field of the Invention
The present invention relates to a semiconductor device including an MEMS (Micro Electro Mechanical Systems) sensor.
2. Description of Related Art
Application of an MEMS sensor to a portable telephone has recently been started, and hence the MEMS sensor attracts much attention. For example, an acceleration sensor for detecting the acceleration of an object is known as a typical MEMS sensor.
FIG. 6 is a schematic sectional view showing the structure of a conventional acceleration sensor.
The acceleration sensor shown in FIG. 6 includes a sensor body 101, a weight 102 held by the sensor body 101 and an annular base 103 supporting the sensor body 101.
The sensor body 101 integrally includes a membrane 104, an annular support portion 105 connected to a peripheral edge portion of a first surface (lower surface) of the membrane 104 and a weight fixing portion 106 connected to a central portion of the first surface of the membrane 104. A plurality of piezoresistors (not shown) are dispersively formed on a second surface (upper surface) of the membrane 104. An annular groove 107 having an isosceles-trapezoidal section narrowed as approaching the membrane 104 isolates the support portion 105 and the weight fixing portion 106 from each other.
The weight 102 is in the form of a disc, for example. This weight 102 is arranged under the weight fixing portion 106, so that a central portion of the upper surface thereof is fixed to the weight fixing portion 106.
The base 103 is in the form of a ring having an inner diameter and an outer diameter generally identical to those of the lower surface of the support portion 105 of the sensor body 101. The support portion 105 is so placed on the base 103 that the base 103 supports the sensor body 101. The weight 102 is provided between the sensor body 101 and a surface on which the base 103 is set in a noncontact state with the base 103 and the support portion 105.
When the weight 102 is shaken in response to acceleration, the membrane 104 so vibrates that stress acts on the piezoresistors provided on the membrane 104. The resistivity of a piezoresistor changes in proportion to stress acting thereon. When a change in the resistivity of each piezoresistor is extracted as a signal, therefore, the acceleration acting on the weight 102 can be obtained on the basis of this signal.
The membrane 104 remarkably vibrates with respect to acceleration in a direction Z orthogonal thereto. Thus, the resistivity of each piezoresistor so remarkably changes that the acceleration in the direction Z can be excellently detected. With respect to acceleration in directions X and Y along the membrane 104, however, the membrane 104 only slightly vibrates to cause only a small change in the resistivity of each piezoresistor, and hence it is difficult to accurately detect the acceleration in the directions X and Y.